This invention relates to cryptographic systems, and more particularly, to systems with remote decryption services that decrypt encrypted messages for recipients who cannot decrypt messages on their own.
A variety of cryptographic techniques are used to encrypt email messages. With symmetric key cryptographic systems, the sender of a message uses the same key to encrypt the message that the recipient of the message uses to decrypt the message. Symmetric-key systems require that each sender and recipient exchange a shared key in a secure manner.
With public-key cryptographic systems, two types of keys are used—public keys and private keys. Senders may encrypt messages using the public keys of the recipients. Each recipient has a private key that is used to decrypt the messages for that recipient.
One public-key cryptographic system that is in use is the RSA cryptographic system. Each user in this system has a unique public key and a unique private key. A sender may obtain the public key of a given recipient from a key server over the Internet. To ensure the authenticity of the public key and thereby defeat possible man-in-the-middle attacks, the public key may be provided to the sender with a certificate signed by a trusted certificate authority. The certificate may be used to verify that the public key belongs to the intended recipient of the sender's message. Public key encryption systems such as the RSA system that use this type of traditional approach are referred to herein as PKE cryptographic systems.
Identity-based-encryption (IBE) systems have also been proposed. As with PKE cryptographic systems, a sender in an IBE system may encrypt a message for a given recipient using the recipient's public key. The recipient may then decrypt the message using the recipient's corresponding private key. The recipient can obtain the private key from a private key generator associated with the recipient.
Unlike PKE schemes, IBE schemes generally do not require the sender to look up the recipient's public key. Rather, a sender in an IBE system may generate a given recipient's IBE public key based on known rules. For example, a message recipient's email address or other identity-based information may be used as the recipient's public key, so that a sender may create the IBE public key of a recipient by simply determining the recipient's email address.
A recipient of an encrypted email message must generally use decryption software to decrypt the message. However, in some environments it may be difficult or impossible to install such decryption software. For example, if a mail recipient is traveling and wishes to access their email through a public Internet terminal, it will not generally be possible for the recipient to download and install decryption software on the public terminal. In many business networks, system administrators place restrictions on which software can be downloaded and installed on the computers of the network. Users are often not permitted to install any software on their computers. Restrictions such as these have served to hinder the widespread use of encryption in email messaging.
It would be desirable to be able to provide ways to decrypt messages for recipients who have not loaded decryption software onto their computers.